This study employs a phase I double-blind, randomized, dose-escalation, placebo-controlled study design, with serious dose-limiting toxicity as the primary endpoint. Medically stable, HIV infected subjects with less than 50 CD4+ T cells/ul and no evidence of serious ongoing opportunistic infections will be enrolled in Part A. Three sequential groups of twelve subjects each will be randomly assigned in a 3a/ manner to receive twice-weekly, subq/ injections of either rhIL-12 or matching placebo for four weeks, as an adjunct to their stable antiretroviral therapy. The primary objection is to determine the tolerance of a range of chronic subcutaneous rhIL-12 dosing regimens in HIV-infected subjects with <50 cD4 Tcells/ul who have no evidence of serious ongoing opportunistic infections, and who maintain concomitant antiretroviral therapy.